I wouldn't consider that proof, I would consider that an anecdotal statement. Your mileage may vary.
Since swap file performance is all about latency, why does it stand to reason that my swap file is not performing best on my WD Raptor drive which is <5ms seek time (ON AVERAGE). That means that it is seeking much faster than any 5400rpm drive given ANY location on the disk, not just at the beginning. 5400rpm drives are slow, have little cache, and have very high latencies.
You may have improved your performance via partitioning, however until I see proof, I do not believe that using a slow obsolete hard drive for the swap file alone is going to help anybody.